1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to electrical switching apparatus, such as circuit breakers, including a trip mechanism. The disclosed concept also relates to trip mechanisms, such as trip units, for circuit interrupters.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices; circuit interrupters, such as circuit breakers; network protectors; contactors; motor starters; motor controllers; and other load controllers. Electrical switching apparatus such as circuit interrupters and, in particular, circuit breakers of the molded case variety, are well known in the art. See, for example, U.S. Pat. No. 5,341,191.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Molded case circuit breakers typically include a pair of separable contacts per phase. The separable contacts may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip unit, which senses overcurrent conditions in an automatic mode of operation. Upon sensing an overcurrent condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
Industrial circuit breakers often use a circuit breaker frame, which houses a trip unit. See, for example, U.S. Pat. Nos. 5,910,760; and 6,144,271. The trip unit may be modular and may be replaced, in order to alter the electrical properties of the circuit breaker.
It is well known to employ trip units which utilize a microprocessor to detect various types of overcurrent trip conditions and provide various protection functions, such as, for example, a long delay trip, a short delay trip, an instantaneous trip, and/or a ground fault trip. The long delay trip function protects the load served by the protected electrical system from overloads and/or overcurrents. The short delay trip function can be used to coordinate tripping of downstream circuit breakers in a hierarchy of circuit breakers. The instantaneous trip function protects the electrical conductors to which the circuit breaker is connected from damaging overcurrent conditions, such as short circuits. As implied, the ground fault trip function protects the electrical system from faults to ground.
Some known molded case circuit breakers (MCCBs) include a short delay time setting. The actual short delay trip time is intentionally delayed and has a minimum trip time of approximately 37 milliseconds resulting from the calculation time of a short delay algorithm performed by a microprocessor. The instantaneous feature of these MCCBs is provided by a fixed analog override circuit. A single zener diode is predetermined with a single fixed threshold value. The fixed analog override circuit detects a peak current value and initiates a trip in less than one line cycle. Because the zener diode is a fixed and non-adjustable component, the instantaneous trip threshold is set to a single fixed value.
U.S. Pat. No. 7,203,040 discloses a circuit breaker and trip unit including an arc reduction maintenance switch for reduction of arc flash energy and the severity of arc flash exposure. Specific trip functions are manually overridden with a maintenance trip function that reduces arc energy should a fault occur. The manually enabled and disabled maintenance trip function reduces the pickup currents of the specified trip functions and/or reduces or eliminates the time delays of the specified trip functions. For example, instantaneous trip is reduced from 10 times to 2 times rated current, short delay pickup is reduced from 8 times to 1.5 times rated current, the short time delay of 0.5 second is reduced to 0.050 second (essentially no time delay), the (equipment protection) ground fault pickup current is reduced from 0.5 times to 0.25 times rated current, and the ground fault time delay is reduced from 0.5 second to 0.050 second (essentially no time delay). As another example, instantaneous trip is reduced from 10 times to 4 times rated current, short delay pickup is reduced from 8 times to 3 times rated current, the short time delay of 0.5 second is reduced to 0.050 second (essentially no time delay), the (equipment protection) ground fault pickup current is reduced from 0.5 times to 0.25 times rated current, and the ground fault time delay is reduced from 0.5 second to 0.050 second (essentially no time delay). A light emitting diode (LED) is energized with the maintenance switch in the maintenance position to alert personnel of the change in the trip function. Other means for signaling the implementation of the maintenance trip functions can be provided such as a text message where the trip unit has a visual display.
A known arc flash reduction mode allows a user to remotely enable a relay inside a trip unit in order to place an instantaneous setting of the trip unit into its lowest maintenance mode position (e.g., 2.5 times the frame rating). This relatively very low setting will allow the circuit breaker to interrupt and clear a fault containing the least possible amount of arc flash energy. This reduction in arc flash energy offers relatively greater protection to personnel working in near proximity to the circuit breaker. When remotely enabled, a blue light emitting diode (LED) on the trip unit faceplate lights when the relay is remotely enabled. This lit LED verifies that the trip unit has been placed into the lowest instantaneous arc flash reduction mode and, thus, that the circuit breaker can be safely approached. However, the LED might become covered over and/or blocked from view if, for example, the circuit breaker is installed into a panel or other electrical enclosure with metal doors that can obstruct the LED or a corresponding display. Maintenance personnel do not wish to open the metal doors to check for the lit LED, since if the LED is off, an arc flash could cause an un-safe condition that might result in serious injury or death. Hence, a solution to this problem is needed.
There is room for improvement in electrical switching apparatus, such as circuit interrupters.
There is also room for improvement in trip mechanisms, such as trip units, for circuit interrupters.